There are several ways you can do this, the main issue though is how long you want to keep remembering that 'last' number.

As a rule of thumb: When dealing with lists the first object to check out is "zl".

In your case you want "zl change"; it accepts a list in both inlets (though using the left is the better idea here) and when it receives the same value / object as before it doesn't do anything. Only if the value is different than the previous value will it output the new value.